Electrodes for use in electrolytic processes have been known which have a base or core metal bearing a layer or coating of metal oxides. The core metal of the electrode may be a valve metal such as titanium, tantalum, zirconium, niobium or tungsten. Where the coating is an oxide mixture, an oxide of the core or substrate metal can contribute to the mixture. As taught for example in U.S. Pat. No. 3,711,385, such mixture can include an oxide of the substrate metal plus at least one oxide of a metal such as platinum, iridium, rhodium, palladium, ruthenium, and osmium.
It has also been known that such mixture which can be termed a noble metal oxide mixture, can be a mixture of ruthenium oxide and iridium oxide. Such have been taught generally in U.S. Pat. No. 3,632,498 and examples shown specifically, when combined with titanium oxide, in U.S. Pat. No. 3,948,751. Particularly for utilization as a coating on an electrode used in an electrolysis of an aqueous alkali-metal halide, e.g., sodium chloride, it has been taught in U.S. Pat. No. 4,005,004 that such noble metal oxide mixture can be particularly serviceable when in further mixture with both titanium oxide and zirconium
The invention is broadly directed to an electrode having reduced oxygen evolution during electrolysis of halogen-containing solutions particularly at low pH, such electrode comprising an electrically conductive metal substrate having a coating of enhanced stability under alkaline conditions, which coating comprises at least 15, but less than 25, mole percent iridium oxide, 35-50 mole percent ruthenium oxide and at least 30, but less than 45 mole percent titanium oxide basis 100 mole percent of the oxides present in the coating. Thereby the coating has a molar ratio of titanium oxide to the total of the oxides of iridium and ruthenium of less than 1:1, and should have a molar ratio of ruthenium oxide to iridium oxide of greater than 1.5:1 and up to 3:1.
In another aspect, the invention is directed to a coating composition adapted for providing the foregoing described mixed metal oxide coating and in a still further aspect is directed to the method of making an electrode which is hereinbefore defined. The electrode will be particularly useful as an anode in a membrane cell used for the electrolysis of brine that is at a pH within the range of from about 2 to about 4.